The relevant section of the law is here. Politicla geeks may recognise it as the same section the Prime Minister invoked in an effort to hide his conversation with John Banks. But as Graeme Edgeler points out:

The factual scenario surrounding the conversation between John Banks and John Key that raised so many thorny questions, seems unlikely to arise here. It was not clear whether Bradley Ambrose intended to record the conversation; nor was it clear whether the conversation was a private communication. I am having some difficulty constructing a scenario in which either of these questions is likely to arise here.

Why? Because all the law requires for an offence is that someone "intentionally intercepts any private communication by means of an interception device". Defences of authorisation are covered in another subsection, and phrased in such a way that they simply do not apply here (as any interception has been admitted as being unlawful). There's no doubt about intention, so there's only the factual question of whether the GCSB's unlawful surveillance involved the interception of private communications (the definition of "interception device" being tautologous, its unnecessary to quibble about that).

can quite easily construct scenarios in which the requirement for knowledge or recklessness as to the lack of authorisation may be difficult to establish beyond reasonable doubt.

The problem is that because of the secrecy surrounding GCSB, we don't know what they did (other than that it was a Section 16 interception because there was no Ministerial warrant), and therefore we don't know what law they may have broken. Plus of course there will be the general reluctance of the police to hold their own, or their allies, to account. But if they fail to do so, they will prove that our legal system is basically dysfunctional, that the law does not apply to all, and that they are nothing but a criminal gang in fancy dress.